epic: resurrect quickstart-openshift-modules and Java Quarkus overlay #489

Summary

Resurrect the repository as quickstart-openshift-modules to serve as a modular overlay source for Backstage software templates. Restores OpenShift templates, purges legacy Helm charts, stabilizes the Quarkus Native (GraalVM) runtime on Java 21 LTS, and establishes the overlay contract with bcgov/quickstart-openshift.

(Note: CI/CD workflow consolidation to bcgov/actions and bcgov/actions-openshift is handled separately).

Sub-Tasks

• feat: restructure repository layout and replace Helm charts with OpenShift templates #486 Restructure repository layout and replace Helm charts with OpenShift templates
• feat(java): align to Java 21 LTS and add local compose dev service #487 Align to Java 21 LTS and add local compose dev service
• feat: establish Backstage overlay contract and verification #488 Establish Backstage overlay contract and verification

Acceptance Criteria

• Directory taxonomy supports composable overlays (backends/java-quarkus, backends/python-fastapi).
• Legacy Helm charts (charts/) are completely removed.
• Quarkus Native builds on Java 21 LTS and runs locally via docker-compose.yml.
• Stamping backends/java-quarkus over quickstart-openshift results in a functional OpenShift-ready application.